The invention pertains to a device and a method for exhaust-gas aftertreatment in an internal-combustion engine. The exhaust gas aftertreatment is effected by selective catalytic reduction of nitrogen oxides from the exhaust gas from an internal-combustion engine operating with excess air by introducing a liquid reducing agent.
The nitrogen oxide emissions from an internal-combustion engine, in particular a diesel internal-combustion engine, operating with excess air can be lowered with the aid of the selective catalytic reduction (SCR) technique, in order to form atmospheric nitrogen (N2) and water vapor (H2O). The reducing agent that is used for the purpose is either gaseous ammonia (NH3), ammonia in aqueous solution, or urea in aqueous solution. The urea serves as an ammonia carrier and is injected into the exhaust system with the aid of a metering system, upstream of a hydrolysis catalytic converter, where it is converted into ammonia by means of hydrolysis, and the ammonia in turn reduces the nitrogen oxides in the actual SCR or deNOx catalytic converter.
The important components of a metering system of this type are a reducing-agent vessel, a pump, a pressure regulator, a pressure sensor, and a metering valve. The pump delivers the reducing agent, which is stored in the reducing-agent vessel, to the metering valve, by means of which the reducing agent is injected into the exhaust-gas flow upstream of the hydrolysis catalytic converter. The metering valve is actuated by means of signals from a control device, in such a manner that a specific, currently required quantity of reducing agent is supplied as a function of operating parameters of the internal-combustion engine (see, German patent DE 197 43 337 C1).
It is an advantage of the ammonia-releasing substances which are present in aqueous solutions, such as for example urea, that storage, handling, delivery and metering are relatively simple in technical terms. A drawback of these aqueous solutions is that, depending on the concentration of the dissolved substance, there is a risk of freezing at certain temperatures. 32% strength urea solution, as is typically used as reducing agent in SCR systems, has a freezing point of xe2x88x9211xc2x0 C. Therefore, devices for heating the metering system have to be provided in order to ensure that all the components of the system are able to function within an acceptable time after the system has been started at ambient temperatures of below xe2x88x9211xc2x0 C. and to prevent system components from freezing during operation.
One of the main components is the urea pressure sensor. Since this pressure sensor continuously monitors the urea pressure system, and in particular the pressure sensor can be used to detect freezing of the reducing-agent pump, of the connecting hoses or of the metering valve, this pressure sensor has to be reliably thawed and kept frost-free. The pressure sensor element is expediently fitted spatially in the vicinity of the control electronics for the metering system and in the vicinity of the pump outlet. Nevertheless, it is difficult if not impossible to thaw the pressure sensor only by means of the heating of reducing-agent line, reducing-agent pump and the inherent heating of the control electronics.
U.S. Pat. No. 5,884,475 (German published patent application DE 44 32 577 A1) discloses a device for avoiding frost damage to parts of an exhaust-gas cleaning installation which operates on the principle of selective catalytic reduction during stationary periods and for allowing such installations to operate below the freezing point of the reducing-agent solution used. For this purpose, the device has a thermally insulated reservoir for the reducing-agent solution and a feedline which is connected thereto and ends in an outlet opening for the liquid, a nonreturn valve, which can be acted on by a pressurized gas, being provided in the feed line. The reservoir and the feed line can be heated by means of an electrical heater which supplies a heat exchanger with heat.
Japanese patent application JP 61073382 describes a method for temperature compensation in a semiconductor pressure sensor, to the diaphragm surface of which a plurality of resistors are applied, which are used for pressure measurement. To eliminate changes in the ambient temperature from the pressure measurement, compensation resistors are provided. These heater elements are used for temperature compensation in the event of a change in ambient temperature and not to heat the pressure-sensor diaphragm. Therefore, these heater elements are applied outside the sensor diaphragm.
It is accordingly an object of the invention to provide a device and a method, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which avoids frost damage to components of an exhaust-gas aftertreatment installation, both during stationary periods and when an installation of this type is operating at temperatures below the freezing point, of the reducing-agent solution used.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for exhaust gas aftertreatment by selective catalytic reduction of nitrogen oxides in the exhaust gas of an internal-combustion engine operating with excess air, comprising:
a reduction catalytic converter for cleaning the exhaust gas;
a reducing agent vessel for storing a reducing agent to be injected into the exhaust gas, and a reducing agent pump communicating with the reducing agent vessel for delivering the reducing agent;
a metering valve communicating with the reducing-agent pump for introducing the reducing agent into the exhaust gas upstream of the reduction catalytic converter in a flow direction of the exhaust gas;
a metering control unit for controlling an introduction of the reducing agent according to demand; and
an electrically heatable pressure sensor for recording a pressure of the reducing agent connected to the metering control unit, the pressure sensor having a pressure-sensor diaphragm and electrical heating resistors for heating the pressure sensor disposed on the pressure sensor membrane.
In accordance with an added feature of the invention, there are provided electrical resistors for recording a temperature of the reducing agent disposed on the pressure sensor diaphragm of the pressure sensor.
In a preferred embodiment of the invention, the sensor membrane is formed of Al2O3 (alumina).
In accordance with an additional feature of the invention, the pressure sensor is arranged in a feed line connecting the reducing agent pump to the metering valve.
In accordance with another feature of the invention, the pressure sensor and the reducing-agent pump are combined to form a structural unit within a common housing.
With the above and other objects in view there is also provided, in accordance with the invention, a method for exhaust-gas aftertreatment by selective catalytic reduction of nitrogen oxides in the exhaust gas of an internal-combustion engine operating with excess air, the method which comprises:
delivering a liquid reducing agent from a reducing-agent vessel with a reducing-agent pump and, under certain operating states of the internal-combustion engine, metering the reducing agent into the exhaust gas upstream of a reduction catalytic converter with a metering valve;
recording a pressure of the reducing agent with a pressure sensor having a pressure-sensor diaphragm; and
upon determining that a temperature of the pressure sensor lies close to or below a freezing point of the reducing agent, heating the pressure sensor with electrical heating resistors disposed on the pressure-sensor diaphragm of the pressure sensor.
In accordance with a further feature of the invention, the temperature of the pressure sensor is recorded with the electrical heating resistors on the pressure-sensor diaphragm. Alternatively, or in addition, the temperature of the pressure sensor is recorded with additional electrical resistors on the pressure-sensor diaphragm.
The idea on which the invention is based is that of using the pressure-sensor diaphragm as a support for heating resistors and for temperature measurement. The technology for production of the sensor resistor network is used to additionally apply heating and temperature-measuring resistors.
The use of the sensor-diaphragm surface for electrical heating of the aqueous urea solution avoids taking up additional space, sealing points and plugs for electrical power supply or control.
The electrical heating resistors enable the sensor diaphragm to be heated directly and therefore enable the availability of the pressure sensor to be ensured as quickly as possible and the pressure of the urea system to be monitored even during the thawing phase.
The additional costs for making the pressure sensor heatable are relatively low, since at most it is necessary to print an additional layer of resistors, and only one additional electrical connection on the sensor element is required.
Since the pressure sensor is in any case electrically connected to the control unit, there are only slight costs for electrical connection of the heating.
If the heating resistors are printed with a thick-film paste with a suitable temperature coefficient, they simultaneously serve as a temperature sensor. Since the pressure sensor is in any case, on account of the temperature compensation, calibrated by laser trimming under controlled temperatures, balancing the temperature sensor entails only insignificant additional costs. On the other hand, if the temperature profile of the pressure sensor is known through measuring of the sensor temperature, it is possible to dispense with temperature compensation, which considerably reduces the overall costs of the sensor. It is also possible for a separate temperature-measuring resistor to be applied for measuring the temperature, which results in higher accuracy of the temperature measurement, since the resistance can be optimized for this application.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device and method for exhaust-gas aftertreatment in an internal-combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.